The present invention relates to plasma tap hole welding, wherein by means of a welding current provided on an electrode and a plasma gas a plasma jet directed onto a workpiece to be welded is generated, the plasma jet at least partly going through or penetrating through the workpiece, wherein at the exit side of the plasma jet at least one electrical conductor is arranged, via which a current, the so-called penetration current, is measured. The penetration current is then the current flowing between the electrode and the conductor arranged under the workpiece.
Welding designates the connecting of workpieces by material engagement under application of heat and/or pressure. Herein, welding additive materials can be used in connection with known welding methods. For metals, usually melt welding methods with heat application are used. These can, however, also be used for welding of glass or for thermoplastic materials.
In fusion welding, welding is usually performed with locally limited melt flow without application of force and thus without pressure. The connecting or bonding of workpieces is usually achieved in form of a welding seam or a welding point in known methods.
An especially interesting group of welding methods is the so-called shielding or inert gas shielded welding. Inert gas shielded welding is differentiated into a number of clearly distinguishable methods. For example, it is referred to metal shielding gas welding (MSG welding), tungsten inert gas welding (TIG welding) and plasma welding. Amongst the inert gas welding methods, plasma welding takes up a special place, as, due to the contraction of the arc, higher energy concentrations of the plasma are achievable compared to many competing methods.
Plasma welding is counted among the tungsten shielding gas methods (TS-methods). Herein, an arc serves as heat source. The plasma jet is generated by ionization of a gas or gas mixture by the arc. The plasma welding methods can be differentiated according to the type of arc, which either burns as so-called non-transmitted arc between a non-melting, negative (tungsten) electrode and the inside wall of a plasma nozzle, or as so-called transmitted arc between the tungsten electrode and the workpiece. The corresponding methods are known as plasma jet or plasma arc welding (PJ or PAW welding). Modern plasma welding facilities often utilize a combination of PJ and PAW techniques, also known as PJAW welding, in which the non-transmitted arc, which is operated with a few amperes, is used as pilot arc, which, i.a. by means of a pre-ionization of the gas distance, makes possible contactless ignition of the main light current. Fed process gases are ionized by means of said arcs, so that a plasma jet directed to the workpiece is formed, which, for example, can be moved along a desired welding seam path. Up to three gases or gas mixtures can be added in plasma torches or burners, for example via nozzle systems concentrically provided around the electrode; these gases comprise the plasma gas, the focusing gas for contracting the plasma jet and the inert gas or shielding gas.
In known methods, the plasma jet and potentially the focusing gas is encased or shielded by shielding gas. The use of shielding gas i. a. serves to protect the melt from oxidation during the welding process.
Plasma tap hole welding is a variant of plasma welding. Plasma tap hole welding is, as a rule, used for sheet thicknesses of up to 8 to 10 mm. The main application areas lie in chemical facility construction, aviation and space industry as well as container and pipeline construction.
In plasma tap hole welding, the plasma jet usually penetrates through the whole workpiece thickness at the beginning of the welding process. Herein, the melt caused by the melting of the workpiece is pushed to the side by the plasma jet. The surface tension of the melt prevents it falling through the tap hole. Rather, the melt flows together behind the thus formed welding eye and solidifies to form the welding seam.
Plasma tap hole welding is thus usually a method or process, in which a non-melting electrode is used, which is concentrically surrounded by nozzles of the plasma torch, via which at least a plasma gas and a shielding gas are provided. By ionization of the plasma gas with the aid of a pilot arc or a high frequency ignition and by contracting the plasma gas with the aid of a cooled nozzle (plasma gas (copper) nozzle) and a potential additional focusing gas a plasma jet directed to the workpiece to be welded, and encased by shielding gas, is formed. This penetrates the whole/complete workpiece thickness, and pushes the melt caused by the melting of the workpiece to the side, wherein by means of the surface tension of the melt it is prevented from falling through the tap hole. The melt flows together again behind the formed welding eye and solidifies to form the welding seam.
Plasma tap hole welding, a high power welding method, allows the welding of larger sheet thicknesses with lesser thermal distortion and at high welding speeds. Presently, it is generally used for the welding connection of chrome-nickel-steels. Furthermore, this technique is used in case special requirements regarding quality of the welding seam in connection with penetration, seam form and seam appearance apply.
As a rule, the welding seam root penetrating or appearing on the rear side of the workpiece during welding in case of plasma tap hole welding is protected from atmospheric gases by for example water cooled forming gas rails and bath supports. At the same time, the melt is supported hereby. For welding processes, in which welding is performed with a crack or fissure, it is often sufficient to provide a grooved rail made of suitable material (depending on the material of the workpiece for example made of copper, aluminum or stainless steel) on the side of the workpiece opposite the welding nozzle, in which the process (shielding) gas penetrating the workpiece is collected. If expedient, an additional shielding gas (forming gas) can also be provided. Apart from preventing the absorption or acceptance of damaging gases by the root as well as its oxidation the shielding gas also provides a cooling and thus lessens the danger of a falling through of the melt. Also, the shielding gas is advantageous in connection with the forming of the welding seam underside.
For the utilization of plasma tap hole welding, the process-safe provision of the tap hole is a necessary condition. To this effect, an exact welding edge preparation, which requires a lot of time, and a positioning of the workpieces as well as an exact observation of the welding parameters is necessary. In case of deviations from the basic or boundary conditions, for example in case of variable crack dimensions or clearance and edge offsets as well as discontinuities in geometry, which cause a variable heat dissipation in the workpiece, insufficient penetration, spillings, burn dents and a sagging of the weld pool can occur. Especially in case of the most often welded non-alloyed and low-alloyed steels, these process instabilities may occur more frequently due to stronger variation of the chemical composition as well as a lower surface tension and viscosity.
The application of plasma tap hole welding is thus, momentarily, only possible under cost and time intensive procedures in connection with preparation and positioning of the workpiece. Furthermore, the stability of the welding process deteriorates in case of higher sheet thicknesses, so that the weldable sheet thickness is limited. The dominating difficulty especially in connection with plasma tap hole welding of a stable tap hole formation thus limits the industrial applicability of this method substantially in this field.
Plasma tap hole welding is usually performed in a fully automated manner, so that the tap hole formation must be monitored in order to achieve a welding as homogenous as possible. To this end it is known to use optical and pneumatic signals of the welding process for monitoring, in that the brightness of the penetrating (i.e. passing through) plasma jet as well as the pressure resulting from its kinetic energy are used for controlling the process. Also, it is known to utilize the electrical conductivity of the plasma jet exiting on the rear side of the workpiece (referred to in the following as exit side of the plasma jet) as an indication of root fusion directly or indirectly via a supplementary AC circuit between a measuring rail (which can, for example, be part of the mentioned forming gas rail) and the workpiece. The information regarding the height of this so-called penetration or root penetration current, which in known studies is only a few microamperes to a few milliamperes, is used in order to keep the tap hole formation constant via variation of the welding current. In order to achieve this, the welding current is set to a basic level, which is raised to a higher value (pulse level) in case of a lesser penetration current, in order to provide the workpiece with more energy, so that the tap hole becomes wider again. As the thermal resilience of the plasma gas nozzle limits the maximum welding current, however, the performance of the plasma burner can not be fully utilized in the basic current phase, as a “reserve” must always be kept for the pulse level. It follows that the maximum welding speed and the weldable sheet thickness is reduced.
Furthermore, “Double-Sided-Keyhole-Welding” is known. Herein, the arc burns between a plasma and a tungsten inert gas (TIG) welding torch. The workpiece is, after penetration of the workpiece, disconnected from the mains or switched to a currentless state. The provision of a TIG welding torch, however, requires more space and the accessibility of the rear side of the workpiece. Furthermore, only a small influence on the welding seam formation can be achieved by means of the workpiece disconnected from the mains.
The present invention aims to provide a method for plasma tap hole welding, by means of which process stability and handling, especially the stability of the tap hole formation, is enhanced and/or the maximum achievable welding speed is increased.